MHC class II tetramers have not been previously used for direct detection and quantification of antigen-specific CD4 T cells from human peripheral blood. The PI has developed a novel approach for the generation of MHC class II tetramers, and the preliminary data demonstrate that such tetramers can be successfully used for the direct detection of pathogen-specific CD4 T cells in human infectious diseases. The approach is based on the expression of MHC class II/CLIP precursors that can be efficiently loaded with any peptide of interest, permitting the generation of sets of tetramers for systematic investigation of CD4 T cell responses in human diseases. Preliminary data demonstrate that this approach can be used to directly identify virus-specific CD4 T cells in patients infected with HIV or hepatitis C virus. When combined with magnetic enrichment of tetramer-positive cells, this technique permits identification of CD4 T cells that occur at low frequencies (1:10[5]). This approach therefore provides an opportunity to study pathogen-specific immunity in patients following bone marrow transplantation. MHC class II tetramers allow analysis of CD4 T cells independent of effector functions and will therefore be used to determine whether deficiencies in immune responses following bone marrow transplantation are primarily due to functional impairment of CD4 T cells. The MHC class II tetramer core will develop tetrameric forms of MHC class II/peptide complexes for quantification, isolation and molecular characterization of both functional and anergic CD4 T cell populations in patients following HSCT. The MHC class II tetramer core has the following principal tasks: (1) to identify suitable pathogen-derived peptides with peptide binding studies, (2) to generate MHC class II tetramers using the established peptide exchange procedure, (3) to expand the repertoire of MHC class II alleles available for tetramer-based analysis. This core will work closely with investigators who will use MHC class II tetramers as part of their research projects and with the immune assessment core.